Study On The Preparation And High-pressure Behavior Of Nano Palladium Hydrides

The unrestrained use of conventional energy has led to global climate anomalies.To develop one or several sustainable clean energy sources has become an important part of the energy strategies of various countries.Hydrogen is an ideal energy carrier that is abundant,clean and renewable.Although hydrogen energy has many advantages,there are still many problems to be solved in the actual transportation and use process.It also brings a series of problems such as uncontrollable security risks and high additional costs.Therefore,it is particularly important to develop a new hydrogen storage method.In 1968 Cornell University solid-state physicist Neil Ashcroft proposed that combining hydrogen with another suitable element would cause"Chemical precompression",possibly at lower pressures brings the hydrogen to a metallized state.Storing hydrogen in a solid state in a hydride not only increases the storage density,but also makes the transportation process safer.Once this theory was put forward,it attracted widespread attention.Metal hydrides,as the most important members of the hydride family,not only possess superior hydrogen storage capacity（In theory,the hydrogen density in metal compounds can reach 253 kg/m³）,but also exhibit desirable properties of high-temperature and even room-temperature superconductivity.For example,the superconducting temperature of H₃S is 203 K at the pressure of 155 GPa,La H₁₀ shows the high-temperature superconductivity of 250 K at the pressure of 151GPa.Recently,Lu-H-N ternary system of light elements rare earth metals showed room temperature superconductivity of 294 K at 1 GPa pressure.The result not only sent shockwaves through the scientific community,but also seems to have brought the future of room-temperature superconductivity industrialization to mankind.Metal hydride is not only a potential superconducting material,but also an important physicochemical model which has been widely studied in many fields such as high-pressure synthesis,catalysis and phase engineering.High pressure hydrogen storage research is a system developed along with the development of hydrogen energy research.Its main goal is to synthesize new hydrogen rich hydrogen storage materials,and try to intercept and apply it to actual production under atmospheric pressure.Palladium hydride（Pd-H）system has good dynamic reversibility.Under the conditions of less than 500 K and several atmospheres,palladium can adsorb a large amount of hydrogen（about 1000 times its own volume）to form Pd H_0.7,and H desorption requires very little energy.Unfortunately,until now,the synthesis of Pd H can only be performed under high pressure and high temperature（HPHT）,and the higher hydrogenometric Pd H₂ or Pd H₃ is limited to theoretical calculations.With the development of materials science and high-pressure technology,more phases can be prepared by nanotechnology,and higher-pressure conditions can be achieved,which provides a prerequisite for the further development of metal-hydrogen systems.Nanostructures provide materials with special intrinsic properties such as high specific surface area,structural defects,metastable structure and lattice deformation.In the process of metal hydrogenation,surface energy of the system can be reduced,entropy loss can be reduced,excess enthalpy in grain boundaries can exist,hydrogen molecular dissociation can be promoted,hydrogenation kinetics can be accelerated,and hydrogen atom permeation path can be shortened.Inspired by the above,the following palladium-hydrogen systems are systematically and deeply studied in this paper,and the results are as follows:（1）We successfully synthesized Pd nano-icosahedron with a particle size of8.3±0.8 nm by an aqueous wet-chemical method,and used it as a starting material to synthesize Pd H at a very low pressure（～0.2 GPa）.Its structural parameters（space group:8）38),a=4.101?)were determined by structural refinement and B-M third-order equation of state fitting.After increasing the pressure to 30 GPa and laser heating,Pd H exhibits preferential orientation of facets（111）.Through computational simulation of the hydrogen desorption process,we give evidence that the（111）facet has a high potential barrier,which can effectively restrict the diffusion of hydrogen atoms,thus achieving the retention of Pd H.This achievement increases the hydrogen content of atmospheric-retainable palladium hydride by nearly 40%,which greatly improves the feasibility of industrial production of metal hydride.It is of great significance to further understand the synthetic pathways and phase transition trends of transition metal hydrides.（2）We use Pd nanoparticles（NPs）with different phases,i.e.,amorphous phase and face-centered cubic（fcc）phase,as starting materials to realize the preparation of distinct Pd H_x materials with high hydrogen ratios under pressure up to 30 GPa with the assistance of laser heating in a diamond anvil cell（DAC）.As a result,six kinds of Pd H_xmaterials,including Pd H_0.706,Pd H,Pd H_1.3,Pd₃H₅,Pd H₂,and Pd H₃,have been identified during the high-pressure progress.It is found that the phase of the starting Pd nanomaterials plays a significant role in determining the structure of resultant Pd H_xmaterials in high-pressure synthesis.The results showed that:（I）:when employing the amorphous Pd NPs as the starting material,a novel Pd H₃ structure with the highest hydrogen ratio among the reported Pd H_x,to the best of our knowledge,can be obtained at～2000 K and under pressure of～32.2 GPa.Impressively,after recovering the pressure and temperature to the ambient conditions,Pd H_1.3 material has been synthesized,which possesses the highest hydrogen ratio compared with the previously reported ambient-stable Pd H_x.（II）:using fcc Pd NPs as starting material leads to the formation of Pd₃H₅after heating to～2000 K under pressure of～33.5 GPa and the subsequent formation of Pd H_0.706 after quenching to ambient conditions.These findings not only break the research bottleneck remaining in the development of Pd H_x with high hydrogen ratio for more than a century,but also demonstrate that the use of metal nanomaterials as precursors in the laser heating-assisted high-pressure synthesis and their rational phase engineering would open a new gateway to the design and preparation of metal hydrides with high hydrogen ratios for promising future applications.